- Author: Robert Sanders
Reposted from the UC Berkeley News Center:
Historical California vegetation data that more than once dodged the dumpster have now proved their true value, documenting that a changing forest structure seen in the Sierra Nevada has actually happened statewide over the past 90 years.
A team of scientists from the University of California, Berkeley, UC Davis and the U.S. Geological Survey compared unique forest surveys collected by UC Berkeley alumnus Albert Wieslander in the 1920s and ‘30s with recent U.S. Forest Service data to show that the decline of large trees and increase in the density of smaller trees is not unique to the state's mountains.
“Older, larger trees are declining because of disease, drought, logging and other factors, but what stands out is that this decline is statewide,” said study leader Patrick McIntyre, who began the research while a postdoctoral fellow at UC Berkeley and now manages biodiversity data for the California Department of Fish and Wildlife. “Forests are becoming dominated by smaller, more densely packed trees, and oaks are becoming more dominant as pines decline.”
The authors found that the density of large trees declined in all regions of California, with declines up to 50 percent in the Sierra Nevada highlands, the south and central coast ranges and Northern California.
“Based on our data, water stress helps to explain the decline of large trees,” McIntyre said. “Areas experiencing declines in large-tree density also experienced increased water stress since the 1930s.”
The increased density of smaller trees is usually attributed to fire suppression statewide, he noted. Scientists debate the cause of the decline of larger trees, which has been observed in other parts of the world as well, but many suspect that larger trees need more water than smaller trees to withstand droughts and disease.
Co-author David Ackerly, a professor of integrative biology, said that stressed forests and the loss of large trees could exacerbate the global carbon situation, especially since many are hoping that forests will soak up more and more fossil fuel emissions.
“There's no question that if you are losing large trees, you are losing the standing carbon in the forest,” he said. “Loss of these big trees and the impact of drought stress become a big concern going forward in terms of its impact on the carbon cycle; they can turn a carbon sink into a source of carbon released to the atmosphere.”
The results may help forecast future forest responses to climate change, and in particular suggest that increased temperatures and changing water availability may lead to large-scale changes in forest composition throughout western North America.
The study was published online this week in the early edition of the Proceedings of the National Academy of Sciences.
Oaks taking over California forests
One change the study observed occurs repeatedly throughout California's history, as documented by paleoclimatic records in pollen, McIntyre said. Oaks are becoming more prevalent, replacing pines. Pines tend to dominate during cooler, wetter periods.
“Our study shows that areas of greater water stress tend to be dominated more by oaks than by pines, a signal we see despite variation in logging and fire around the state,” McIntyre said.
The study might never have happened if Wieslander's data, stored both in Sacramento and at Berkeley, had not been saved several times from the trash bin, said co-author Maggi Kelly, a UC Berkeley cooperative extension specialist and professor of environmental science policy and management (ESPM). Wieslander acquired the vegetation data while he worked for the California Forest Experiment Station, a Berkeley outpost of the U.S. Forest Service and the forerunner of UC Berkeley's Department of Forestry, now part of ESPM.
“This is really an astonishingly broad and detailed depiction of vegetation in California at that time and it's important that through its nearly 100-year life it has almost been lost a number of times,” she said. “Patrick's is one of the largest and most comprehensive looks at this historic data set in comparison to comparable contemporary data.”
Most of the plots, maps and photos have been digitized thanks to efforts by Kelly, co-author James Thorne of UC Davis and campus librarians who saw future value in the data. Digitization and the study were funded by the Keck Foundation through the Berkeley Initiative on Global Change Biology (BiGCB), as part of an ongoing effort to create an ecological informatics engine, or EcoEngine, for analyzing historical digitized data relating to ecological change.
“All these records are now brought together in digital form in the EcoEngine, which will allow more people to plumb the data and ask more questions, such as, What about logging? What do the photographic records show?” Kelly said. “We need to remember that there are a lot of valuable collections of data that we can use to make inferences about the future.”
Other co-authors are Christopher Dolanc of UC Davis and Alan and Lorraine Flint of the USGS California Water Science Center in Sacramento.
- Author: Bill Tietje
Bill Tietje is a UC Cooperative Extension area natural resources specialist in the Department of Environmental Science, Policy and Management at UC Berkeley. He is based in San Luis Obispo.
UC Cooperative Extension's Master Gardeners have received many calls during the past few months concerning the poor condition of many California native oak trees, in both urban and rural landscapes. Many evergreen oaks, including coast live oaks, have brown leaves and thin foliage. Adding to the unattractiveness, a deciduous oak, the blue oak, dropped its leaves ahead of schedule. Although a tree may look unhealthy, it can recover.
Early leaf drop is a deciduous tree's adaptation for conserving water that it otherwise would lose through transpiration from its leaves, which can occur as long as the leaves are green.
More recently, another deciduous oak, the valley oak, kept its brown, dead leaves longer than usual. This could be due to the virtual lack of rainfall and wind last fall and early winter, both of which typically contribute to an earlier leaf drop.
So why are these things happening?
As you know, it's dry out there! In fact, the past 12 months have been the driest on record, going back to 1870. Not surprisingly, many oaks are under water stress—and they show it.
This situation reminds one of the conditions during the drought of 1988-1990, one of the most widespread and severe droughts in the state's history. Coincidentally during that time in three counties on the Central Coast, UC Cooperative Extension was conducting a study that included the monitoring of coast live oak, blue oak, and valley oak trees on study plots scattered throughout Monterey, San Luis Obispo and Santa Barbara counties. Many of the oaks looked stressed. Some of the trees succumbed to the drought. Small oak trees in the undergrowth and on steep terrain with southern exposure, and shallow, infertile soil, were most vulnerable. Such sites are typically drier than other slopes and orientations. However, large, mature trees—or, large branches on these trees—on more gentle slopes, also died. Usually there is not only a single factor that causes the decline and mortality of oak trees. Drought stress lowers the trees' defense, making the trees more susceptible to mortality factors such as decay fungi and boring beetles. Most likely the drought caused early death of some oak trees that would have persisted otherwise.
What can be done?
Surely our native oaks have been through droughts before. So the oak trees, other than the very small or very old trees, should be okay. Nonetheless, given the very low rainfall this year it may be prudent to give a valued tree in the urban landscape a “deep watering”.
A deep watering can be accomplished by moving a hose around under the tree's canopy during the day for a day or two at a low flow or a trickle stream, such that the water percolates into the soil, not simply run down the hill. Water a few feet away from the base of the tree to avoid inviting damage from crown rot caused by the fungi Phytophthora cinnamomi. Water-saturated soil increases the chances of infection of the tree trunk.
A deep watering followed by soil drying for a month or two should not harm the tree. In fact, a deep watering may be the best recommendation for invigorating your thirsty oak tree, thus providing some insurance that the tree will survive this current drought.
I should mention that unless California receives normal or better rainfall the rest of the rainfall season, it is likely that early leaf drop will occur next summer. Remember, as suggested above, the early browning and fall of leaves does not mean that your tree will die. This is simply the tree's way of adapting to conserve water when soil moisture is low. Unless the tree is severely weakened by some other cause, it will leaf out normally the following spring.
For more information: Tietje, W., W. Weitkamp, W. Jensen, and S. Garcia. 1993. Drought takes toll on Central Coast's native oaks. California Agriculture 47(6):4-6.